It would suffice to keep up the full number of a tree, which lived on an average for a thousand years, if a single seed were produced once in a thousand years, supposing that this seed were never destroyed and could be ensured to germinate in a fitting place; so that, in all cases, the average number of any animal or plant depends only indirectly on the number of its eggs or seeds.
In looking at Nature, it is most necessary to keep the foregoing considerations always in mind--never to forget that every single organic being may be said to be striving to the utmost to increase in numbers; that each lives by a struggle at some period of its life; that heavy destruction inevitably falls either on the young or old during each generation or at recurrent intervals.
Lighten any check, mitigate the destruction ever so little, and the number of the species will almost instantaneously increase to any amount.
The causes which check the natural tendency of each species to increase are most obscure.
Look at the most vigorous species; by as much as it swarms in numbers, by so much will it tend to increase still further.